Wales is in the grip of a measles epidemic, thanks to the anti-vaccine scare more than a decade ago. Once the critical mass of herd immunity dropped below a certain threshold, in came the old, deadly -- and utterly preventable -- disease.

Redditor Jasonp55 has a neat demonstration of the perils of confusing correlation with causation, and his well-chosen example makes this a potentially useful chart for discussing this issue with friends who won't vaccinate themselves and their kids.